Washing systems using dry solvents such as carbon dioxide have been known for several years. In recent years this technology has become more popular mainly due to the environmental advantages compared to other washing systems using different solvents. Known dry cleaning systems usually contain a cleaning chamber wherein e.g. fabrics are cleaned, a distiller for separating the carbon dioxide from contaminants so that the carbon dioxide can be reused, a storage tank for storing the carbon dioxide when not in use for cleaning, a cooling unit, and a compressor for moving solvent in the system and building up a pressure in the system.
One example of such a system is disclosed in EP 1842602, which discloses a multiple bath CO2 system wherein the system and method is designed for processing parts in more than one bath of dense phase carbon dioxide. The system disclosed in EP1842602 is designed to work within a temperature interval between 10-20° C. which corresponds to a pressure around 58 bar.
Currently the trend in this technology goes towards increasing the pressure in these systems in order to achieve advantages in the cleaning properties of the solvent being used. Even though EP 1842602 disclosed that higher temperatures and pressures could be used, there is no teaching how this can be done.
One known way to increase pressure is to use two compressors as illustrated in FIG. 1. When using a multi-stage gas compressor in a CO2 washing machine there is a need of intercooler, because when transporting the gas the gas pressure is increased and the gas temperature increase almost exponential, and might reach levels that are critical for the compressor so there is a risk that the compressor is damaged.
Thus, FIG. 1 of the present application, illustrates a system having two compressors wherein the second compressor (3) is used to further increase the pressure after a first compressor stage (2) has compressed the gas once. In order to avoid this temperature increase, a cooling unit (23), which is an air cooled intercooler, having flanges and a fan (5), has been arranged between the first (2) and second (3) compressor stages as illustrated in FIG. 1.
However a drawback with the prior art cooling system is that it is rather complex with moving parts, and that it needs a power source of electrical energy in order to function. Another drawback with the prior art systems is that the cooling unit needs to be controlled. The fan is either constantly operating, or the fan has to be synchronized with the compressor so that it runs when the compressor is operating. None of the solutions are good, since the fan will either constantly use electrical energy or it needs to be controlled, still needing electrical energy when operating.
In general, a system being more complex and having more mechanical moving parts is more likely to fail and thus shortens the time between maintenance, which in the end may lead to periods wherein the system can not be used, and even further in the long run increased costs. Another drawback with such prior art system is that the cooling unit in it self is very expensive which increases the total cost of the whole system.
A further drawback with prior art systems is that fluid stored in the storage tank is supercooled in order to avoid a too high pressure in the storage tank. During operation of the system this may become a problem because when the system is running the fluid is cooled between each washing cycles when transferred back to the storage chamber, this leads to a too cool fluid (supercooled fluid) in the storage tank, so that when the fluid is transferred to the next washing cycle the fluid does not have optimal temperature for washing.